On IdeaScale @kier replied to out suggestion and posted the keyboard shortcuts for taking screen snapshots on the M models.

A/D INPUT + PAGE up = sub display (creates MSNAPS_%number%.png)

A/D INPUT + PAGE down = main display (creates MSNAPM_%number%.png)

The trailing 'S' of MSNAPS refers to the sub display and the trailing 'M' of MSNAPM is for the main display.

Thanks to @kier for posting this!